Gas-filled electric cable with paper and polystyrene insulation



Aug. 25, 1953 E. L. DAVEY f GAS-FILLED ELECTRIC CABLE WITH PAPER AND PoLYsTYRENE INSULATION Filed Sept. 27, 1950 2 Sheets-Sheet l l- Swen/vo dona/croie 2- Mama/Z50 P4PE@ MP5 5- COPPE/e 72H75 Inventor Attorney E. L. DAVEY Aug. 25, 1953 GAS-FILLED ELECTRIC CABLE WITH PAPER AND POLYSTYRENE INSULATION 2 Sheets-Sheet 2 Filed Sept. 27, 1950 INVENTOR. Edward Les/ie Davey BY (JM/4&1 /7a z/u? #g4/1,60.,

H/.S` A TTORNE YS Patented Aug. 25, 1953 GAS -FILLED ELECTRIC AND POLYSTYRE CABLE WITH PAPER NE INSULATION Edward Leslie Davey, Hale, England, assigner to W. T. Glover & Company Limited, Manchester, England, a British company Application September Z7 In Great Britain 2 Claims. 1

In the specification of our prior patent application No. 40,450, now abandoned, we have described and claimed a supertension electric cable having a lapped dielectric built up of an inner part consisting of oriented polystyrene and an outer part of pre-impregnated paper. The wall thickness of the oriented polystyrene portion lies within a range whose lower limit is equal to 10% of the radius of the conductor surface wtih a minimum thickness of fourteen lappings and whose upper limit amounts to 45% of the radial wall thickness of the whole dielectric. Under operating conditions the interstices of both parts of the cable are filled with gas under superatmospheric pressure.

The minimum thickness that is specified in terms of numbers of layers or lappings will only be greater than 10% of the conductor radius in the case of cables operating at the lower end of the supertension voltage range and having conductors at the lower end of the range of practical conductor sizes, or in other cases, Where thick tapes of oriented polystyrene are used. As will be explained more fully later in the specification, a minimum thickness of the wall of polystyrene of fourteen layers is required in order to ensure that any inaccuracies in lapping do not affect the dielectric strength of the wall to a serious extent and thus allow excessive electrical stress to be imposed on the impregnated paper beyond.

In the aforesaid application We have indicated that a coating of lubricant such as an oil or petroleum jelly may be applied to the surfaces of the strips of polystyrene film before they are wrapped on in place in the cable. This coating ensures sufficient freedom of relative movement between the superposed layers of polystyrene nlm for the normal bending of the cable during the subsequent manufacturing stages and during the laying operation.

We have now found that such a coating of lubricant may not always suffice to prevent the sticking together of the strips of polystyrene film after the cable has been put into service. The temperature developed in the cable conductor or conductors when the cable is under load tends to cause the neighboring strips of polystyrene film to stick together despite their initial separation by a lm of lubricant. The reduction in the flexibility of the cable caused by the sticking together of the strips of polystyrene film is not of material consequence after the cable has been installed unless the installation is only temporary. However the sticking together of the strips 1950, Serial No. 187,040 October 13, 1949 of polystyrene nlm has the disadvantage that gaps having larger radial dimensions than those corresponding to the radial thicknesses of the strips are liable to be formed. Such gaps reduce the electrical efliciency of the dielectric and also to some extent block the gas iiow paths within the inner part of the dielectric, thus rendering the dielectric less satisfactory if the gas pressure in any part of the cable is reduced, as by the occurrence of a leak.

By the present invention we provide a modied form of the cable described and claimed in the specification of the aforesaid patent which eliminates or at least reduces to a considerable extent the sticking together of the strips of oriented polystyrene lm of which the inner part of the dielectric is built up. In this modified form of cable the inner part of the dielectric is built up of alternate single strip layers of oriented polystyrene lm and layers, preferably single strip layers, of pre-impregnated paper. As a result each strip of polystyrene is separated from each of its neighboring strips of polystyrene by pre-impregnated paper. By pre-impregnated paper we mean paper impregnated before the lapping process with a cable impregnating compound that does not flow at any temperature within the working range of the cable.

Figure 1 is a perspective view of the stepped end of a length of cable embodying the invention, and

Figure 2 is a diagrammatic drawing showing fragmental sections of the inner portions of a lapped cable dielectric, the sections being taken at longitudinally spaced points in the cable.

An example of a single core cable having a dielectric built up in this way is shown in Figure l which is a perspective view of a stepped end `of the cable. The cable is designed for a working voltage of 132 kv. between the conductors of a B-phase A. C. transmission system. It will be seen that the cable comprises a stranded conductor l, the circumferentially disposed wires of which are flattened as by drawing the conductor through a die, the diameter of the drawn conductor being 2.0 cms. The conductor is provided with a screen 2 consisting of two layers of pre-impregnated perforated metallized paper tape applied with a 5 to 10% overlap with their metallized faces outwards followed by a similar lapping of pre-impregnated unperforated metallized paper tape. These tapes are three mils thick so that the screen has a radial thickness of 0.023 cm., giving the conductors an effective external radius of 1.023 cms. and a smooth surface in contact with the surrounding dielectric.

The dielectric consists of an inner part 3 and an outer part 4. The inner part 3 consists of alternate single strip layers 3a of pre-impregnated paper of 2.5 mils (0.006 cm.) thickness and single strip layers 3b of oriented polystyrene film of 4 mils (0.01 cm.) thickness built up to an external radius of 1.403 cms., the rst layer being of polystyrene, the next paper, the third of polystyrene and so on. Alternatively the rst layer may be of paper, the second of polystyrene, the third of paper, and so on. The outer part 4 of the dielectric is built up entirely of helical lappings of strips of pre-impregnated paper to an external radius of 2.27 cms. Over` the outer part of the dielectric is a screen 5 formed by a helical lapping of copper tape 3 mils thick applied with a to 20% overlap. The screened core is enclosed in a lead or a lead alloy sheath 6 which is mechanically reinforced to withstand an internal gas pressure of about 14 atmospheres by helical lappings l, 8 and 9 of metal tape. Enclosing these reinforcing tapes is a second lead or lead-alloy sheath l0 which is protected against corrosion by an outer protective covering Il of bitumen-impregnated jute or hessian or the like. Under operating conditions the interior of the cable, namely the portion within the inner sheath B contains nitrogen under a pressure of 14 atmospheres.

When operating at a line voltage of 132 kv., the working stresses in the specific example of cable described above are approximately as follows:

Kv./cm. At the conductor surface-in the paper 85 At the conductor surfacein the polystyrene 115 At the outside of the inner part of the dielectric-in the paper 62 At the `outside of the inner part of the dielectricin the polystyrene 84 At the inside of the outer part of the dielectric 62 At the outside of the outer part of the dielectric 38.4

However, the electrical stress limitations in supertension cables are not imposed by the working voltage but by the impulse or surge voltages to which they may be subjected in service. These surges, which arise from switching operations or from lightning eiects on parts of the circuit exposed to atmospheric influences, are unidirectional in character and consist of voltage waves of amplitudes several times that of the power frequency waves and of duration of the order of from 10 to 100 microseconds. Under these conditions the gas-filled spaces within the dielectric may ionize and thereby increase the stress on the solid portion of the dielectric. The amount of this increase will depend upon the selected build-up of the dielectric wall and the accuracy with which that build-up can be attained. In a taped conductor dielectric, the joints between the successive turns of tape in each layer are staggered with respect to those of the adjacent layers. The amount of stagger may be expressed as of the width of a tape, in which case the dielectric can be seen to be formed of a number of superposed groups of n, layers. 20,11 0f which pings.

groups is an approximate repetition of the underlying group as regards the location of the helical gas spaces in the group. With accurate lapping the effective strength under surge conditions of each group is of that of a convolute wrapping of the same wall thickness. Should however two spaces in two successive layers of a group coincide at any point due to inaccurate application of the tapes or, as a result of a tape breakage, should the radial thickness of the space at the break be doubled by making a lap joint between the broken ends, the strength of the group will be reduced from If such a fault occurs in the polystyrene layer, the stress across the impregnated paper beyond the fault will be increased. It will be apparent that, from the point of view of reducing the increase of stress caused by the inadvertent or necessary superposing of two neighboring gaps, n should be as large as possible. It is however limited by the need for adequate lateral spacing of the gaps in adjacent layers. In practice a spacing of 25% of the width of the tape is desirable giving n a value of 4. With perfect lapping at such spacing the effective strength under surge conditions of the polystyrene portion of the dielectric will be of a convolute wrapping of the same wall thickness. The effect of a coincidence of two butt spaces in two successive layers will be to reduce this gure of '75% as will be more easily appreciated from a consideration of Figure 2 which of course is not to scale. The tapes of oriented polystyrene, of which the thickness has been greatly exaggerated, are designated A1, A2, A3, A4, B1, B2, B3, B4 and so on and the gas spaces, which may ionize under surge conditions, are designated X. An enlarged space is shown at X1 in the right hand section where the second layer butt spaces inadvertently coincide with those of the underlying layer. This will reduce the effective strength of the polystyrene portion of the dielectric to an extent dependent upon the radial location of the coincident spaces and upon the number of layers. To avoid overstressing the pre-impregnated paper part oi the dielectric under surge conditions, the effective surge strength of the polystyrene portion should, inpractice, be about of the optimum value of '75%. It will be apparent that with fourteen layers the effective strength of the polystyrene wall will be about 85% of the optimum value of 75%, it being assumed that the chances of a second imperfection in the second and third groups occurring in radial alignment with an imperfection in the first group are negligible. It will also be perceived from the sections to the left that the effective strength will not fall below this figure when the enlarged rspace X1 occurs at any other point in the polystyrene wall although it may do so in some cases if the polystyrene wall is composed of less than 14 lap- For this reason a minimum thickness of the wall of polystyrene of 14 lappings is specified and this applies to all spacing within the limited practical range.

Under a surge o f ten times the working voltage, the maximum stress in the polystyrene would be 1150 kv./cm., which is well below 1250 kv./cm., the surge breakdown stress of the polystyrene iilm. The cable is therefore more surge resistant than cable having a dielectric of the same radial dimensions built up wholly7 of pre-impregnated paper for under similar surge conditions the stress in the paper at the surface or the conductor of such a cable would be 910 kv./cm. Which is greater than 800 kv./cm. which is the surge breakdown strength of pre-impregnated paper when used alone. It may be thought that under such surge conditions the stress in the paper at the surface of the conductor of the modied cable wuold exceed the surge breakdown strength of pre-impregnated paper. This is not so however. Due to the impedance of the paper decreasing as the breakdown voltage strength is approached there is a tendency for the stress on the paper layers to be shunted to the neighboring polystyrene layers, so that the actual maximum surge stress is not 850 kv/cm. as would appear from the data given above but a lower Value which is less than the surge breakdown strength of 800 kv./cm. Thus the modified cable is more surge resistant than an all paper insulated cable of the same dimensions and is comparable as regards its surge strength with a cable having a dielectric consisting of an inner part built up entirely of polystyrene lm and an outer part built up of pre-impregnated paper, in accordance With the invention disclosed in the specification of our prior patent application No. 40,450.

What I claim is:

1. A super-tension cable comprising a conductor, a plurality of helical lappings of insulating material on said conductor forming an intersticed laminated body of dielectric surrounding said conductor, a gas-impervious enclosure surrounding said body of dielectric, and under operating conditions gas under super-atmospheric pressure filling the interstices in said laminated body, said laminated body comprising an inner part built up of alternate single strip layers of oriented polystyrene film and layers of pre-impregnated paper, and an outer part built up of layers of pre-impregnated paper, said inner part comprising at least fourteen layers and having a radial wall thickness of not less than 10% of the radius of the conductor surface and not more than of the combined radial Wall thickness of said inner and outer parts.

2. A super-tension cable comprising a conductor, a plurality of helical lappings of insulating material on said conductor formingan intersticed laminated body of dielectric surrounding said conductor, a gas-impervious enclosure surrounding said body of dielectric, and under operating conditions gas under super-atmospheric pressure lling the interstices in said laminated body, said laminated body comprising an innel part built up of alternate single strip layers of oriented polystyrene lm of which the thickness is about 4 mils and layers of pre-impregnated paper of which the thickness is about 2.5 mils, and an outer Ipart built up of layers of pre-impregnated paper, said inner part comprising at least fourteen layers vand having a radial Wall thickness of not less than 10% of the radius 0f the conductor surface and not more than 45% of the combined radial Wall thickness of said inner and outer parts.

EDWARD LESLIE DAVEY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,068,940 Wiseman Jan. 26, 1937 2,093,411 Bowden et a1 Sept. 21, 1937 2,102,974 Robinson Dec. 21, 1937 2,289,734 Scott et al. July 14, 1942 2,309,992 Scott et al. Feb. 2, 1943 

1. A SUPER-TENSION CABLE COMPRISING A CONDUCTOR, A PLURALITY OF HELICAL LAPPINGS OF INSULATING MATERIAL ON SAID CONDUCTOR FORMING AN INTERSTICED LAMINATED BODY OF DIELECTRIC SURROUNDING SAID CONDUCTOR, A GAS-IMPERVIOUS ENCLOSURE SURROUNDING SAID BODY OF DIELECTRIC, AND UNDER OPERATING CONDITIONS GAS UNDER SUPER-ATMOSPHERIC PRESSURE FILLING THE INTERSTICES IN SAID LAMINATED BODY, SAID LAMINATED BODY COMPRISING AN INNER PART BUILT UP OF ALTERNATE SINGLE STRIP LAYERS OF 